Xerox copiers and laser printers have become an essential and almost indispensable part of today's office environment. With the advent of rapid technology growth and information explosion, the desire to provide improved copiers and/or printers always presents a constant challenge.
Both the xerox-type copiers and the laser printers involve the technique of xerography, which, in essence, is an electrophotographic process. The key element in the xerographic technique is an electrophotoreceptor, or photoreceptor, which is an optical element electrically insulative in darkness but becomes electrically conductive after exposure to selected light beams. Photoreceptors play key roles in all the six main steps in the electrophotographic process: charging, photocharging, imaging, image transfer, development, and cleaning. In order to obtain high-quality printed images, photoreceptors should have high charge acceptance, low dark conductivity and high sensitivity. Another requirement that arises as a result of the substantially increased speed in today's xerox copiers and laser printers is that the photoreceptor must be able to retain its quality after a large number of repeated use.
A photoreceptor is a material which is capable of generating and transporting electron-hole pairs after absorbing photon particles. Photoreceptors are typically classified as inorganic or organic types. Examples of inorganic photoreceptors include: selenium, cadmium sulfide, zinc oxide and amorphous silicon. Examples of organic photoreceptors include: photoconductive polymers such as poly-N-vinylcarbazole and polyvinylanthrancene; low molecular weight organic compounds such as carbazole, anthracene, oxadiazole, certain hydrazones and certain polyarylalkanes; organic pigments or dyes such a phthalocyanine pigment, azo pigment, cyanine pigment, polycyclic quinone pigment, perylene pigment, indigo dye, thioindigo dye and squaraine dye, etc. Due to their advantages of low production cost, non-toxicity and high flexibility in utilization, organic photoreceptors (which are sometimes called organic photoconductors, or OPC) have largely replaced inorganic photoreceptors as the predominant photoreceptors among the commercialized photoreceptors.
Photoreceptors may also be classified according to their structures into three main types: (1) mono layer type photoreceptors, such as that disclosed in U.S. Pat. No. 3,484,237, (2) functionally separated laminated type photoreceptors, such as those described in U.S. Pat. Nos. 3,837,851, 3,850,630, 4,123,270 and 4,293,628, and (3) microcrystalline distribution type photoreceptors. The functionally separated laminated type photoreceptors typically comprise a conductive substrate, a charge generation layer (CGL) and a charge transport layer (CTL). Optionally a barrier layer or an adhesive layer may be sandwiched between the conductive substrate and the charge generation layer. In the production of the photoreceptors of the functionally separated laminated type, a charge generation layer comprising a charge generation material and a polymeric binder is coated on a conductive substrate, and then a charge transport layer comprising a charge transport material and another polymeric binder is coated thereupon.
Organic photoreceptor may be produced by selecting among a variety of suitable charge generation materials, suitable charge transport materials and suitable polymeric binders. U.S. Pat. No. 4,150,987 (the '987 patent) disclosed hydrazone-containing charge transport layer to be included in a layered electrophotographic plate. The hydrazone compounds disclosed in the '987 patent generally suffered low sensitivity problems. In U.S. Pat. No. 4,666,809 (the '809 patent), hydrazone compounds represented by two general formulas were disclosed for use in a lamination type of photosensitive member having improved sensitivity. The hydrazone compounds disclosed in the '987 patent involve relatively complicated molecular structure and are expensive to manufacture. U.S. Pat. No. 4,957,836 (the '836 patent) disclosed a new type of hydrazone compounds with simpler molecular structure; however, these hydrazone compounds exhibited undesirable crystallinity problems. U.S. Pat. No. 4,830,944 (the '944 patent) attempted to enumerate an exhaustive list of hydrazone compounds that may be used as charge transport material in electrophotographic process. However, undue experimentation would be required to sort out hydrazone compounds that may be suitable for the intended use. Furthermore, despite of the lengthy enumeration in the '944 patent, extensive research effort is still needed to develop new hydrazone compounds that could either provide improved properties over the prior art compounds or overcome some of the problems presented in the prior art charge transport materials.